1. Field of the Invention
The present invention relates generally to image reading apparatuses and, more particularly, to an image reading apparatus having improved shading correction.
2. Description of the Related Art
As image input means of computer or original image reading means of digital copiers or facsimile devices, image reading apparatus has conventionally been used which reads a still image of an original by an image sensor, then subjects resultant image data to various image processings and outputs an image signal.
An optical system of such an image reading apparatus includes below a platen glass a light source for illumination, a lens array for converging reflected light from the original, and a one-dimensional image sensor (line sensor) including solid image sensing elements formed of CCDs (charge-coupled devices) or the like, arranged in a main scanning direction (a lateral direction). Moving the line sensor in a sub-scanning direction (a longitudinal direction) causes an original image to be fractionized into pixels and then read.
A photoelectric conversion signal output from the line sensor is quantized and converted into image data of, e.g., 8 bits (256 gradations) by analog/digital conversion means. After that, the image data is subjected to various image processings, so that an image signal to be transmitted to an image forming apparatus such as a printer is generated.
In image reading apparatus, uniforming of image data (level matching) called a shading correction is carried out in order to correct dispersion of image data due to differences in sensitivity between any image sensing elements of the line sensor, light intensity distribution (unevenness in quantity of light) of the light source in the main scanning direction, distortion of lens or the like.
That is, a reference image of uniform density is read prior to reading of the original, and reference image data corresponding to pixels of one line or correction data corresponding thereto is stored. In reading of the original image, image data of each line is corrected for each pixel by repetitively using the reference image data of one line or the correction data for each line.
Conventionally, when suddenly produced external noise such as switching noise of a power source or crosstalk noise produced between any signal transmission paths is superimposed on a photoelectric conversion signal, reference image data corresponding to unspecified pixels (normally one or two pixels) in a single line becomes abnormal data. Then, image data is corrected in accordance with the reference image data including the abnormal data.
Thus, when the original image is reproduced by an output of the image reading apparatus (image data provided after the shading correction), abnormal data portion appears in the form of striped image noise in the subscanning direction, resulting in a degradation in quality of an image.